Histogenesis, series of organized, integrated processes by which cells of the primary germ layers of an embryodifferentiate and assume the characteristics of the tissues into which they will develop. Although the final form of the cells that compose a tissue may not be evident until the organ itself is well along in development, distinctive biochemical reactions, which are the signatures of histogenesis, can be detected much earlier.

Histogenesis can be detected at both the cellular and tissue level. The gradual conversion of an early mesodermcell into a muscle cell is an example of histogenesis at the cellular level. Most often, however, single cells do not undergo histogenesis but change only when they are part of a larger group of cells acting at the tissue level, as when thousands of cells aggregate to form the tissue of the islets of Langerhans in the pancreas. It is this tissue that produces insulin. The transformation of a mass of undifferentiated cells into an organ is known as organogenesis.

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irregularly shaped patches of endocrine tissue located within the pancreas of most vertebrates. They are named for the German physician Paul Langerhans, who first described them in 1869. The normal human pancreas contains about 1,000,000 islets. The islets consist of four distinct cell types, of...

in embryology, the series of organized integrated processes that transforms an amorphous mass of cells into a complete organ in the developing embryo. The cells of an organ-forming region undergo differential development and movement to form an organ primordium, or anlage. Organogenesis continues...

...The formation of organs, even those of diverse function, shares some common features, which are considered in this section. As the organs form, so do their component tissues, in a process termed histogenesis.